This invention relates generally to the design of valves and, more particularly, to the design of a combination check valve and pressure relief valve.
In the art, check valves and pressure relief valves are known. Generally, a check valve functions to restrict flow in one direction while a relief valve is used to regulate flow pressure. Furthermore, U.S. Pat. No. 4,948,092 discloses a combined check valve and pressure relief valve having a resilient duckbill valve body. Fluid passing through a cylindrical core around a valve actuator functions to open the lips of the duckbill valve body to permit the free flow of the fluid while back pressure functions to seal the lips of the duckbill valve. Manual depression of the valve actuator, however, causes the valve actuator to penetrate and open the valve lips to selectively permit backflow to provide the relief valve function.
A further combination check valve and pressure relief valve is shown in prior art FIG. 13. As illustrated, the combination check valve and pressure relief valve includes a check compression spring 78 and a relief compression spring 81. The check compression spring 78 and relief compression spring 81 cooperate with a valve stem 82, a machined valve seat 83, and valve plug assembly 80 to provide the check valve and pressure relief functions. To establish the opening characteristics of the valve, a wet set procedure is utilized which involves setting the valve in a test stand and using fluid to activate the valve. The valve configuration is then adjusted, for example by turning a set screw, until the valve shows the desired amount of opening, based either on flow, or pressure, or both. While this wet set process works for its intended purpose, it does suffer the disadvantages of being time consuming and expensive. The use of a machined valve seat also increases the relative cost of the valve while further disadvantageously limiting the physical characteristics that can be provided to the valve seat. Still further, the large size of the compression spring 81, disadvantageously requires the machining of holes in the valve seat to provide a means for relief fluid flow (since the size of spring 81 provides no fluid flow passages through spring 81). The size of spring 81 additionally increases the overall size of the valve assembly thereby preventing use of this valve in applications such as integrated hydrostatic transaxles.
To overcome these, and other disadvantages, a combination check valve and pressure relief valve is provided for use in regulating the flow of fluid between a first fluid side and a second fluid side. The combination check valve and pressure relief valve includes a valve plug, a valve guide moveably positioned in relation to the valve plug, a valve stem engaged to the valve guide, a check compression spring attached to the valve stem and positioned between the valve guide and the valve plug, a valve seat carried by the valve stem, and a relief compression spring positioned between the valve seat and the valve guide. The valve guide is adapted to move the valve stem relative to the valve seat against the force of the relief compression spring to allow fluid to pass from the first fluid side to the second fluid side through a fluid flow passage formed between the valve stem and the valve seat. Furthermore, the valve seat, valve stem, and valve guide are adapted to move relative to the valve plug against the force of the check compression spring to thereby remove the valve seat from an opening formed between the second fluid side and the first fluid side to permit fluid to pass through the opening.